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Requirements for increasing safety and reliability: new design of Current Transformers and experience on multiple stress test on composite bushings

A. DI GIULIO

D. FALORNI

A. FRAIOLI

V. IULIANI Terna Rete Italia

Italy

M. SVANBERG

ABB

Sweden

M. GULLO

STRI

Sweden

P. CARDANO

ALSTOM Passoni e Villa

Italy

M. DE NIGRIS

G. PIROVANO

RSE

Italy

vincenzo.iuliani@terna.it

Introduction

Some failures of HV equipment and transformers that occurred a few years ago, led Terna to devote the maximum effort to improve the safety of substations.

Terna therefore carried out an extensive campaign in search of solutions to reduce the risk linked with the potential explosion of porcelain insulated equipment, the projection of equipment pieces, and to increase at the same time the overall reliability of the substations. .

As for the risk of explosion, the research was directed toward the possibility to replace oil with SF6 in equipment internal insulation while, for the risk of projection of pieces of equipment (including the porcelain housing) in case of explosion, it was decided to adopt equipment housings made of polymeric materials.

Terna has started to install exclusively SF6 type HV circuit breakers since the early 80’s and, since 2003, has equipped circuit breakers with the overpressure valves and polymeric external insulation, to further improve their safety characteristics.

As far as voltage transformers (VTs) are concerned, it was not possible to reduce dramatically the risk of explosion, as data transmission requires the use of capacitive VTs and as the present constructive technology of voltage dividers is based on the use of dielectric oil. The porcelain external insulation of VTs, has systematically been replaced with polymeric insulation.

CTs with SF6 insulation were introduced in Terna since 1998 but, at that time, insulating oil was preferred because of the risk of possible leakage of SF6 equipment, triggering the necessity of putting out of service an entire substation bay to avoid the risk of failure of the component. The project discussed in this paper was carried out in view of eliminating the drawbacks related to the use of oil and porcelain insulation in CTs, thus enhancing their safety and reliability. The activity was conducted involving two major European equipment manufacturers.

Transformer bushings are purchased since 2010 only using external polymeric bushing and the RIP technology.Before proceeding to a large-scale deployment, Terna carried out a series of tests with STRI and RSE to assess the effectiveness of the solutions envisaged and to verify if the improvement introduced, in addition to ensuring greater safety in case of failure, did not reduce the performances of the equipment in terms of operation and reliability, with special reference to its behavior under different environmental conditions.

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New design of Current Transformers

The high destructive failure rate detected on oil insulated CTs operating in their substations and the increase of line loads and environmental average temperatures in many locations, especially in the southern part of Italy, has carried Terna to develop a technical specification for design, construction, testing and delivery of a new CT’s type, called "High reliability HV CTs" (TA ad “affidabilità incrementata” – TA a.i.). Fig.1

Considering ten years experience in substations O&M and the inevitable complaint for restrictions and incompatibility with other equipment whereby CTs must interoperate, Terna has attended to this problem, adopting the following new technical features in order to improve the equipment reliability:

Internal insulation: the use of SF6 instead of oil to avoid, in case of internal fault, the explosion of insulating casing with projection at distance of any parts thereof, affecting the electrical service and especially the safety of operating staff eventually present nearby.

External insulation: the use of composite insulators instead of ceramic ones, with the following benefits:

a) greater intrinsic safety in case of internal fault

b) high mechanical resistance

c) high ratio between mechanical resistance and weight

d) high temperature resistance

e) high dielectric strength

f) greater reliability in polluted environments

g) greater behavior in case of seismic activity

h) excellent self-extinguishing Fig.1

i) maintenance free due to the features that don’t require periodic washing.

The required percentage of silicon rubber within the composite insulator is greater than 30%. This noteworthy feature ensures excellent performance during the tracking and erosion tests and multiple stresses 5000 h, based on IEC standards Fig.2

Fig.2

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Gas tightness:

j) increase to avoid outages and environmental risks minimizing, in case of SF6 leakages (maximum permissible leakage rate ≤ 0.1% per year)

k) use of double O-ring seals with hi performance features (-30°C ÷ +85°C) Fig.3

l) Greased to avoid the corrosion of the surface of the flanges and protect also the seals from external environment

m) low roughness of the seals seat

n) introduction of strictly condition for leaks during type and routine tests (tightness test with helium gas).

Fig. 3

Operating conditions:

a) use in high temperature areas (special operating conditions according to IEC 60044-1: -25 / +55 °C, considering the solar radiation effect of 1000 W/m2)

b) insulation class provided for the windings, corresponding to a overtemperature not lower than 50 K

c) temporary primary current overload possibility (120% permanent; 150% for 1 h)

d) increase of withstand voltages at power-frequency and impulse (BIL), compared to values provided by standards for various levels of nominal voltage (i.e.: for 420 kV CTs, the LIWL is 1550 kV, the SIWL is 1175 kV and the power frequency withstand voltage is 680 kV).

e) internal insulation reinforced to ensure a temporary operation even in the absence of SF6 gas pressure (1 h at 1,1 Um/√3 with P = 0 MPa rel.)

f) internal arc withstand for 0,5 s at rated short-circuit current expected, without projection of material, according to GIS requirements

g) seismic qualification performed with AF5 Level, independently of the installation site, on the entire system "metal support/CT”

h) rupture disks with high reliability in order to avoid their unexpected interventions

i) high quality materials with resistance to harsh environmental conditions (systematic use of aluminium and stainless steel materials)

So Terna has required for majors manufactures to develop a new CT’s project meets the requirements of technical specification, with evidence of all solutions and technical improvements adopted.

The first prototypes of TA a.i., made by the manufacturers, were tested and validated by Terna. All technical specification and standards type tests have also been run under the supervision of Terna technicians. Finally, TA a.i. have been manufactured and supplied in such quantity in order to satisfy the Terna oil insulated CT’s replacement long-term plan.

Currently, Terna has installed about 2500 TA a.i.in replacement of many oil insulated CTs. About 3000 oil insulated CTs, whose disposal has been scheduled in the coming years, remain to be replaced.

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Experience on multiple stress test on composite bushings

It’s known that, although polymeric insulators are safer than ceramic ones (no shattering in case of internal fault/explosion), if not proper polymer materials are used their life cycle could be shorter than ceramic ones because polymeric material may deteriorate due to environmental and electrical causes as shown in Fig-4

Fig.4

Due to this reasons, Terna wanted to be sure of the reliability of bushings with polymeric insulator before proceeding to a large-scale deployment.

IEC 62217 standard “Polymeric insulators for indoor and outdoor use with a nominal voltage >1 000 V – General definitions, test methods and acceptance criteria” describes some tests to assess the quality of material (e.g. hardness test, flammability test and so on). Previous edition of the standard (2005-10) contained a multiple (combined) test as one of three possible alternatives for Tracking and Erosion tests, together with 1000 hours Salt Fog and Tracking Wheel tests. In the current edition of the standard only the 1000 hours Salt Fog test is included, while the Multiple Stress test and Tracking Wheel tests has been moved to a new document, IEC Technical Report 62730 (2012-03). Terna opinion is that the Multiple Stress test, with some modifications, can be a valid alternative to “field” test stations, e.g. “Koeberg Insulator Pollution Test Station (KIPTS)” of ESKOM Electric Utility, Cape Town (South Africa).

Terna was not completely satisfied with the results of tests performed on HV CTs polymeric insulator samples and decided to perform two partially different tests introducing some deviations referred to previous international standard. Previous test showed actually an abnormal tracking/erosion phenomena even before the end of the test (one specimen was completely damaged already after 4000 h). This test had two deviations from the standard; the salinity was lower than prescribed (2 kg/m³ instead of. 7 kg/m³) and the test objects had longer creepage distance (1200 mm instead of maximum 800 mm).

Due to this experience and to the following discussion with bushing manufacturers and laboratory engineers, Terna decided to perform multiple stress test on bushing samples with some deviation from the standard.

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The present paper describes two different tests, performed respectively at STRI (located in Ludvika, Sweden) and at RSE (located in Milan, Italy) institutes.

The tests were performed in the framework of the current contract for the supply of autotransformers to Terna. They were, anyway, performed only for investigative scope. According the agreement between Terna and manufacturers, in case acceptance criteria in accordance to IEC 62217 - 2005 Annex A would be fulfilled, final test report would have stated this. While, if the criteria would be not fulfilled, the report would only have stated the detailed result of the test (presence of tracking or erosion and the severity etc).

In both experiences described below no electrical discharge occurred, the visual inspection after 5000 hours did not reveal any tracking, erosions reaching the glass-fibre core or sheds puncture. Therefore the acceptance criteria are satisfied.

It would have been more useful, of course, to perform both different tests on same specimens to compare them: perhaps it will be done in future. Anyway Terna thinks that current results (of different tests on different specimens) are very interesting for itself and for the possible future use of a no standardized test.

Test on ABB specimens carried out in STRI Lab (Ludvika)

The test was performed in the period November 2011 – June 2012.

The test has been performed with the following modality, some of them agreed with Terna, are not referred to the standard:

• Test specimens had longer creepage distance than prescribed by the standard, approximately 1100 mm instead of maximum 800 mm.

• The test specimens were rotated after every 1000 h (180°, 90°, 180° and 90°). • The salt fog had an initial salinity of 14 kg/m³ (twice std value). • The salt fog was generated by means of turbo sprayers mounted at the lower part of the

chamber directed upwards towards the roof of the chamber and calibrated by measuring the precipitation per hour over a minimum period of 16 hours according to IEC 62217 – 2005 clause 9.3.3.1.1.4. This is called indirect spray.

• If more than one flashover occured (regardless over which test object) the salinity would be decreased according to following table; the insulators would be washed by tap water and the test resumed with the new salinity.

• The voltage during the test was approximately 32 kV at 50 Hz, thus the tested specific creepage distance was 20 mm/kV. The test voltage was determined in accordance with IEC 62217 – 2005 Annex B and based on the creepage distance of the tested insulators.

• Measurement of the wettability of the insulator surface according to IEC TS 62073, method C - The spray method (former STRI Hydrophobicity Guide) and visual inspection were performed before the test, after every 1000 hours of the test, and after the test, together with continuous leakage current measurements during the 5000 hours.

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• The test layout with the deviations from the standard marked in red is showed in Fig.5

Fig.5

• The standard requires that 2 identical specimens have to be tested. In STRI test 2

different types of hollow insulators were tested, i.e. 4 specimens in test room.

Test specimen type 1 Test specimen type 2

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In the following final considerations about test: The insulators showed excellent performance during the test. The salinity was kept constant at 14 kg/m³ during the entire test since no single flashover occurred (as previously described, the initial intention was to reduce the salinity if flashover would occur during the test). All test objects fulfilled the acceptance criteria’s and passed the test. After 5000 hours no signs of tracking, erosion or puncture of the silicone housing were found on the test objects. After the 5000 hours tracking and erosion test the surface of the silicone housing was covered by a thin pollution layer consisting of dissolved salt and crystallized salt. However, the surface of the housings showed still some hydrophobicity with a wettability class of 2-5. In general, service experience of silicone rubber apparatus and hollow insulators with larger diameters than line insulators is positive [1]-[3]. This is represented by the results of Multiple Stress test with indirect spraying. However, the multiple stress test as it is described now in IEC/TR 62730 “HV polymeric insulators for indoor and outdoor use tracking and erosion

testing by wheel test and 5 000h test” seems quite unfavourable for test objects with a relatively large diameter due to the way the salt fog is generated, with IEC 60507 nozzles directed towards the test objects. The IEC 60507 salt fog nozzle distributes the salt fog at an area that is proportional to the distance of the nozzle. Due to the limited space in the test chamber the nozzles are relatively close to the test objects resulting in a quite small salt fog distribution area. With larger test objects, typically hollow insulators for apparatuses, the salt fog area is smaller than the insulator area, resulting in an uneven salt distribution on the test objects. As a result, STRI experienced permanent difficulties with calibration of direct spraying according to formal IEC procedure. Also, sometimes heavy erosion was observed on the prototypes of hollow insulators with large diameters in the laboratory test, which was not observed in service. It is proposed that IEC should summarize the experience of different HV laboratories through the world with present IEC multi-stress test and possibly re-consider its application for insulators with large diameters.

Test on Alstom specimens carried out in RSE

The tests were carried out according to the following procedure. It must be noted that part of the procedure differed from that of International Standards, and was agreed with TERNA: • Test specimens had a creepage distance of about 620mm, thus fulfilling the range span

specified by the Standard (i.e. 500mm < l < 800mm). • The salt fog had an initial salinity of 14 kg/m3 (i.e the double of the value specified in the

Standard). • In case of occurrence of more than one flashover (regardless over which test object), in the

occasion of any event, all the insulators under test had to be washed by tap water, and the test resumed with a new level of salinity, according to the following table:

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• The salt fog was generated by means of spray nozzles of the IEC 507 type, positioned in accordance with IEC 1109 Annex C, and directed torwards the walls and the roof of the test chamber.

• The salt fog was calibrated according to IEC 62217 – 2005 clause 9.3.3.1.1.4, i.e by means of measuring the precipitation per hour over a minimum period of 16 hours. The precipitation was of the order of 0,4±0,1 lh/m3.

• The Standard requires that 2 identical specimens be tested. During the research, 2 different types of hollow insulators were tested, i.e. 4 specimens were present in the test room. The test layout adopted and the actual test arrangement is shown schematically in the Fig.6 here below.

Layout of test arrangment

Fig.6

• Two specimens were rotated of 90° around their axis after every 1000 hours of test. The other two were kept in their original position.

• During the test, the voltage was maintained stable at approximately 17,9 kV at 50 Hz. The test voltage was determined in accordance with IEC 62217 – 2005 Annex B, taking into account the creepage distance of the tested insulators.

• The measurement of the level of hydrophobicity (IEC TS 62073, method C) and the visual inspection were carried out before the test, after every 1000 hours, and at the end of the the test.

• Continuous leakage current measurements were conducted during the 5000 hours. • In case of damage of any of the test object at a degree impairing its capability to withstand

the test voltage at the lowest salinity (7 kg/m3), the specimen was deemed to be kept in the test hall, without any voltage applied. The test sequence was continued on the remaining objects.

The result of this long test has been very satisfying: despite the salt fog conditions heavier with respect to the standard values (14 kg/m3 instead of 7 kg/m3), all the specimens withstood without any flashover the entire test sequence.

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An example of the trend of the leakage currents monitored during the test is shown in the following Fig.7 referred to insulators n. 1 and 2, and to to the portion of the sequence from 2000 h to 3000 h.

Trend of the leakage currents and applied voltage

Fig.7

The surface at the end of the test was found still hydrophobic and without any tracking, puncture or corrosion damage as can be seen in the following Fig-8.

Hydrophobicity and appearance of the insulator after the test

Tracking and erosion test. Test at multiple stresses

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Fig.8

The insulators surface was found covered with a layer of crystallized salt distributed unevenly along the sheds (fig. 9); this deposit was removed easily after washing.

Salt deposit on the insulator sheds after the test

Fig.9

At the end of the test, the appearance of the surface of the samples which were rotated was compared with that of samples kept in the same position during the entire sequence. No important difference was visible on the surface of the insulators, thus reinforcing the fact that the specimens kept fixed (and thus subjected to a more prolonged stress along the side), could withstand without problem the entire test sequence.

The only outstanding difference in the test sample setup with respect to the IEC Standard 62217, was the orientation of the nozzles which were directed toward the walls and the roof and not toward the insulators. This configuration was used to produce a more uniform salt fog on the insulators, according to the requirements of IEC 1109 (standardised tests on insulators for overhead lines considering multiple stress) This choice was made to prevent a very fast (and absolutely not meaningful) damage of the insulators produced by a strong direct flow of salt fog. The adopted configuration better represents actual conditions in the field.

References:

[1] I. Gutman, T. Hayashi, A. Phillips, A. Pigini, J. Seifert, F. Schmuck, M. R. Shariati, V. Sklenicka: “Guide for the Assessment of Composite Insulators in the Laboratory After Their Removal From Service”, Working Group B2.21, CIGRE TB 481, December 2011

[2] I. Gutman, D. Windmar, A. Holmberg: “Pollution and Ageing Performance of Composite Apparatus Insulators with Different Profiles: Service/Field Experience & Laboratory Testing”, World Congress & Exhibition on Insulators, Arresters & Bushings, Seoul, Korea, 17-20 April 2011

[3] I. Gutman, C. Ahlholm, U. Åkesson, A. Holmberg, D. Wu, L. Jonsson: ”Long-term service experience and inspection results of HV equipment made of silicone rubber insulators”, CIGRE Symposium, Auckland, New Zealand, September 14-20, 2013, A3-412